A comprehensive review is presented of reported aspects and putative mechanisms of sleep-like motility rhythms throughout the animal kingdom.It is proposed that ’rapid eye movement(REM)sleep’ be regarded as a special case of a distinct but much broader category of behavior,’rapid body movement(RBM)sleep’,defined by intrinsically-generated and apparently non-purposive movements.Such a classification completes a 2×2 matrix defined by the axes sleep versus waking and active versus quiet.Although ’paradoxical’ arousal of forebrain electrical activity is restricted to warm-blooded vertebrates,we urge that juvenile or even infantile stages of development be investigated in cold-blooded animals,in view of the many reports of REM-like spontaneous motility(RBMs)in a wide range of species during sleep.The neurophysiological bases for motorically active sleep at the brainstem level and for slow-wave sleep in the forebrain appear to be remarkably similar,and to be subserved in both cases by a primitive diffuse mode of neuronal organization.Thus,the spontaneous synchronous burst discharges which are characteristics of the sleeping brain can be readily simulated even by highly unstructured neural network models.Neuromotor discharges during active sleep appear to reflect a hierarchy of simple relaxation oscillation mechanisms,spanning a wide range of spike-dependent relaxation times,whereas the periodic alternation of active and quiet sleep states more likely results from the entrainment of intrinsic cellular rhythms and/or from activity-dependent homeostatic changes in network excitability. A comprehensive review is presented of reported aspects and putative mechanisms of sleep-like motility rhythms throughout the animal kingdom. It is proposed that ’rapid eye movement (REM) sleep’ be considered as a special case of a distinct but also broad category of behavior , ’rapid body movement (RBM) sleep’, defined by intrinsically-generated and apparently non-purposive movements. Study a classification completes a 2 × 2 matrix defined by the axes sleep versus waking and active versus quiet .Although ’paradoxical’ arousal of forebrain electrical activity is restricted to warm-blooded vertebrates, we urge that juvenile or even infantile stages of development be investigated in cold-blooded animals, in view of the many reports of REM-like spontaneous motility (RBMs) in a wide range of species during sleep.The neurophysiological bases for motorically active sleep at the brainstem level and for slow-wave sleep in the forebrain appear to be remarkably similar, and to be subserved in both cases by a pri mitive diffuse mode of neuronal organization.Thus, the spontaneous synchronous burst discharges which are characteristics of the sleeping brain can be readily simulated even by highly unstructured neural network models. Neuromotor discharges during active sleep appear to reflect a hierarchy of simple relaxation oscillation mechanisms, spanning a wide range of spike-dependent relaxation times, of the periodic alternation of active and quiet sleep states more likely results from the entrainment of intrinsic cellular rhythms and / or from activity-dependent homeostatic changes in network excitability.